Induction lamp



Dec. 18, 1956 F. N. c. HANSEN 2,774,905

INDUCTION LAMP Filed July 8, 1955 United States PatentO 2,774,905INDUCTIoN LAMP Frithjof Niels Carl Hansen, Beverly Farms, Mass., as-

signor to Sylvania Electric Products luc., Salem, Mass., a corporationof Massachusetts Application July 8, 1955, Serial No. 520,767 Claims.(Cl. 313-161) This invention relates to electric lamps, and especiallyto lamps in which light is emitted by an incandescent body.

Such lamps have previously been either of the pointsource arc-heatedtype or of the wire filament type, neither of which is satisfactory foruse when a uniformly illuminated field is desired for projectiondevices, iilm printers and the like.

At present, incandescent lamps are generally used for such purposes, thelamp having a series of parallel filaments arranged side-by-side in aplane. In an optical system, the image produced by such a series offilaments is obviously non-uniform in brightness, and in order to reducethe non-uniformity, the system has to be defocussed, that is, the imageis focussed at a point other than that at which the light is to be used.For example, in a film printer, the socalled hlm gate is the position atwhich the light is desired, but the image of the filament has to befocussed at a position beyond the gate, so that the image issufficiently lout of focus at the gate to blue it and give a moreuniform light pattern.

Such defccussing, however, entails a considerable loss of light. Forreasonable uniformity, the loss can be as much at 73% in a conventionalprojector system, leaving only about 27% of the light available at thefilm gate. Thus the major portion of the light is lost. For anincandescent filament source, therefore, the ratio of the amount oflight available to the amount which can actually be used is about 3.7. Alight source with the same total light output and the proper size andshape with uniform brightness could deliver 3.7 times as much light atthe iilm gate. if the source were circular in shape and the film gatesquare, about 40% of the light would be lost in the part of the imageoutside the circumscribed square, but such a source would still deliver2.2 times as much light as a filament source.

ln addition to the loss from the amount of defocussing required, thereis an additional loss with incandescent filament sources due to theirinability to iillthe lens of the optical system completely, because ofthe non-uniformity of the source. The nearer this filling is achieved,the greater will be the light output from the system. A source ofuniform brightness will accordingly givea considerable gain in effectivelight output over that from sources hitherto used.

A light source of uniform brightness throughout its frontal luminoussurface, that is, throughout the surface designed for Aexposure to alens, can be achieved by using as the light source an inductivelyheatedbody of refractory material, such as, for example hafnium carbide,tantalum carbide or niobium carbide, as shown in a copending applicationSerial No. 520,718, filed on July 8, 1955, by Sandford Christopher Peek,lr. For convenience, the inductively heated body is referred to as atarget The target can be heated inductively from a coil placed around itand carrying high frequency current. A field concentrator between thecoil and the target is very effective in improving the eiciency ofenergy conversion to the target. For example, a concentrator has beenused in the form of a metal cylinder closed at one end except for anopening in which the target is placed. The cylinder is slottedlongitudinally along its side, the slot extending radially into theclosed end, in order to eliminate 2,774,905 Patented Dec. 18, 1956 2 anydirect loop around the intensifier itself, transverse to the field.

Prior to my invention, the frontal surface of the target was set flushwith the plane of the frontal surface of the annular concentrator disc,or flush with the frontal edge thereof. I have found, however, thatunder such a condition, the extreme outside edge of the light pattern isless bright than that from the remainder of the frontal surface. ldiscovered that the difference in brightness between the extreme outsidecircumferential edge and the remainder of the face could be eliminatedif the target were set so that its frontal surface was a short distancein front of the plane of the concentrator disc, if the latter were flat,or in general a short distance in front of the front edge of the insideopening in the concentrator disc.

The loss in brightness of the circumferential edge of the target whenthe latter is not positioned according to my invention appears to be dueto the formation of a smooth shiny surface on the cylindrical sides ofthe target in the neighborhood of the central opening in theconcentrator disc. The smooth shiny surface has poorer radiatingcharacteristics than the remainder of the target surface, and alters theheat distribution. By projecting the target a short distance in front ofthe concentrator disc, the smooth shiny surface is confined to a regionfarther from the front circumferential edge of the target, and theradiation characteristics and brightness of the front surface of thetarget become uniform. The target extends into the opening in theconcentrator disc, and in general will extend back to the back edge ofthe annular opening in the concentrator disc.

Fig. l is a perspective view of one embodiment of the invention; Fig. 2is a profile section of the device of Fig. 1; and Fig. 3 is a crosssection through the middle of the device.

ln Figure l, the glass envelope 1 includes the plane glass disc 2, of atype generally called an optical at, through which light from thecylindrical target 3 can emerge when the latter is heated by the passageof a high frequency current in coil 4, which is external to the lamp andnot an integral part thereof.` The target 3 is set in the centralopening 5 of an annular metal disc 6, and is of smaller diameter thansaid opening. The

ice

j disc 6 has the radial slot 7 so that there will not be a completeconductive turn in which current can flow by induction. The disc 6 actsas a closure member for one end of the cylindrical metal shell 8, whichextends longitudinally back in the envelope 1, preferably for a distanceequal to the length of the coil 4 and in register therewith. The slot 7extends from disc 6 longitudinally down the wall of cylinder 8, toprevent the cylinders acting as a complete conductive turn, that is, asa so-called shorted turn. The disc 6 and shell 8 act to concentrate themagnetic field in the vicinity of the target.

The ltubes 9, 9 should not extend across the slot 7 at any point, for ifthey did, they would short circuit it. In cases where liquid cooling isnot desired, the concentrator disc 6 and shell 8 can be supported insome other manner than by tubes 9,9, for example by being supported fromthe glass walls of envelope 1.

The cylinder S is support-ed by the hollow metal tubes 9, 9 which areattached to the copper thimbles 10, 10 at the external ends 11, 11thereof, said copper thimbles being sealed to the bottom wall 12 of theglass envelope 1 in a manner customary in the art of such seals, whichare generally called housekeeper seals.

As shown in Figure 2, the hollow tubes 9,9 terminate in the customarytype of metal connectors 13, 13 used for sealing copper tubing together.Inside the envelope 1, the tubes 9, 9 extend around and across the innerWall 14 of cylinder 7, being attached thereto by soldering or in someother convenient manner. y

The hollow tubes 9,9 are not electrical lead-in conductors, but aremerely conductors for the circulation of a cooling liquid such as water.if the water supply 1s grounded, however, the tubes 9, 9 and thecylinder S and discV 6' to" whichl they are thermally connected, willalso be at ground potential.

The thickness of target 3 is greater than that of the disc 6, and thetarget is placed so that part of it extends above the plane of the topof said disc. In one example, the target thickness was about 25132 inchand the disc thickness about 1/16 inch. The disc was about 2 inches indiameter. Beyond the :gg inch thickness of the target itself, the targetpiece was machined down to a diameter of about 1/16 inch to provide aprojecting support i5 for the main portion of the' target 3, saidsupporting extending about 5%; inch back from said target itself. Thediameter of the target 3 itself was 5/12 inch.

The bottom end of projecting support i5 is .fitted into u zirconiasupporting tube 16 at a considerable distance, about 1A inch in oneexample, from the main Itarget 3, in order to reduce heat losses. Thezirconia tube extends downward along the axis of cylinder '7, and itslower end 17 is held in collar 18 by set screws i9, supported from thelower en'd of cylinder 7 in the bracket 2i), shown in Figure 3 held tosaid cylinder by the screw 25, or in some other convenient manner. lOther refractory materials than zinconia can be used, the zirconia beinggiven as an example.

The whole internal unit or mount is thus supported from the Housekeeperseal thimbles l0, liti, set in the glass base of header 2?. The seals tothe thimbles ifi, l@ can therefore be made before the glass header 21 issealed to envelope l, the sealing being accomplished in a mannerwell-known in the art.

After sealing the header 21, to envelope li, the lamp can be exhaustedin a manner usual in the art, through the metal exhaust tube 22 which isbrazed to the side of one ofthe thirnbles 10. The tube can then befilled with argon at about 11/2 atmospheres absolute pressure, forexample, and the exhaust tube 22 then sealed off to complete the lamp.The exhaust tube 22 being of metal, for example copper, can be flattenedtogether at one part of its length to seal the lamp from the exhaustsystem, and then cut off at the flattened part. Such seals are wellknownin the art.

in operation, one of the tubes 9 is connected to a water inlet byconnector 13, and the other tube 9 is connected to a Water outlet by itsconnector. Water, or other cooling iluid, is then circulated through thetubes 9, thereby keeping the disc 6 and shell S cool. A high frequencysource of say 4 megacycles although other frequencies can be used, isconnected to coil 3, through which high frequency current willaccordingly flow. The magnetic eld inside said coil is concentrated, bydisc 6 and shell 8, around the refractory target 3, which is accordinglyheated by the currents induced in it. The target 3 becomes very hot, itstemperature rising to about 3600" i if sufficient energy is supplied.

rlhe light from the front surface of target 3 then travels through theoptical flat 2 to the plane in which it is to be used.

The embodiment described is merely by way of illustration, and theinvention is not limited to that embodiment. Many modifications will beapparent to those skilled in the art, without departing from the scopeand spirit of the invention. For example, the metal carbide targets canbe heated by means other than induction if desired.

What I claim is:

l. An electric induction lamp comprising` a magnetic field concentrator;a refractory induction target in operative relation therewith to beheated by the magnetic field of said concentrator, the field intheimmediate region of the concentrator deteriorating the surface thereofin said region during operation of the lamp, said target extending awayfrom said yconcentrator for a distance greater than that over whichappreciable deterioration occurs.

2. An electric induction lamp comprising a magnetic field concentrator;a refractory induction target in operative relation therewith to beheated by said field, said target having a uniformly rough surface andthe field in the immediate region of the lconcentrator being such as tosmooth the roughene'd surface of the target in said region, said targetextending away from said concentra-tor for a distance greater than thatover which appreciable smoothing of said target surface occurs.

3. An electric induction lamp comprising: a magnetic field concentratorincluding a disc having an opening therein; and a refractory inductiontarget extending within said opening but spaced therefrom and projectingbeyond the rim of the inner opening in the direction of useful lightemission from the target.

4. An electric induction lamp comprising: a magnetic field concentratorincluding a at disc having an opening therein; and a refractoryinduction target extending within said opening but spaced therefrom andprojecting beyond the plane of the disc in the direction of lightemission from the target.

5. An electric induction lamp comprising: a magnetic field concentratorincluding a disc having an opening therein; and a refractory inductiontarget extending within said opening but spaced therefrom and projectingbeyond the rim of the inner opening in the direction of light emissionfrom the target, whereby the apparent brightness of light emission fromsaid target is substantially constunt over the lateral surface of saidtarget on the side from which useful light is emitted.

6. An electric` induction lamp comprising: a magnetic field concentratorincluding a disc having an opening therein; and a refractory inductiontarget extending Within said opening but spaced therefrom and projectingbeyond the rim of the inner opening in the direction of useful lightemission from the target, a iight-transmitting envelope enclosing saidconcentrator and said target, and a filling of gas in said envelope.

7. An electric induction lamp comprising: a magnetic field concentratorincluding a dat disc having an opening therein; and a refractoryinduction target extending within said opening but spaced therefrom andprojecting beyond the plane of the disc in the direction of lightemission from the target, a light-transmitting envelope enclosing saidconcentrator and said target, and a filling of gas in said envelope.

8. An electric induction lamp comprising: a magnetic field concentratorincluding a disc having an opening therein; and an induction target of arefractory material selected from the group consisting of fantalumcarbide, zirconium carbide, hafniurn carbide7 said target extendingwithin said opening out spaced therefrom and projecting beyond the rimof the inner opening in the direction of useful light emission fro-m thelight.

9. An electric induction lamp comprising: a magnetic field concentratorincluding a fiat disc having an opening therein; and an induction targetof a refractory material selected from the group consisting of tantalumcarbide7 zirconium carbide, hafniurn carbide, said target extendingwithin said opening but spaced therefrom and projecting beyond the planeof the disc in the direction of light emission from the target.

l0. An electric induction lamp comprising: a magnetic field concentratorincluding a disc having an opening therein; and a refractory inductiontarget extending within said opening but spaced thenefrom and projectingbeyond the rim of the inner opening in the direction of useful slightemission 'from the target, said target not extending beyond the rim ofthe inner opening in the direction o pposite to that of useful lightemission from the target.

References Cited in the file of this patent UNITED STATES PATENTS2,171,359 Gefrier Aug. 29, 1939

